FIG. 1 of JP3446685 (Patent Document 1) illustrates the structure of a piezoelectric sound generating element according to the related art in which a diaphragm has a circular profile and a piezoelectric element has a circular profile. In addition, FIG. 7 of Patent Document 1 illustrates the structure of a piezoelectric sound generating element according to the related art in which a diaphragm has a quadrangular profile and a piezoelectric element has a quadrangular profile. In the structure of the piezoelectric sound generating element according to the related art illustrated in FIG. 1 of Patent Document 1, the distance between the profile of the piezoelectric element and the profile of the diaphragm is constant. In the structure of the piezoelectric sound generating element according to the related art illustrated in FIG. 7 of Patent Document 1, the profile of the piezoelectric element and the profile of the diaphragm are parallel with each other at all locations. Therefore, with such structures according to the related art, it is difficult to disperse resonance and make sound pressure frequency characteristics as flat as possible.
FIG. 3 of JP2005-311679A (Patent Document 2) illustrates the structure of a piezoelectric sound generating element according to the related art in which a diaphragm has a quadrangular profile and a piezoelectric element has an octagonal profile. In the piezoelectric sound generating element according to the related art, the profile of the piezoelectric element and the profile of the diaphragm are parallel with each other in some portions, and are not parallel with each other in other portions. The piezoelectric sound generating element described in Patent Document 2 provides sound pressure frequency characteristics that are flatter than those of the piezoelectric sound generating element described in Patent Document 1.
FIGS. 14 and 15 of JP3360558 (Patent Document 3) illustrates a piezoelectric sound generating element in which a piezoelectric element has a circular profile and a diaphragm has a non-quadrangular profile obtained by cutting off a part of a rectangular profile.
FIG. 6 of JP2004-221903A (Patent Document 4) illustrates a piezoelectric sound generating element in which a piezoelectric element with an elliptical profile is affixed to a diaphragm with a quadrangular profile to form a piezoelectric vibrating body, which is covered with a resin sheet (a member that is deformed according to vibration of a power generating vibrating body) that is larger than the piezoelectric vibrating body, the resin sheet having a fixed periphery.